1. Field of the Invention
The present invention relates to an image forming apparatus which jets liquid droplets to form an image.
2. Description of the Related Art
There has been conventionally known an image forming apparatus which includes a head provided with a plurality of nozzles for jetting liquid droplets and which forms an image on a recording medium by jetting the liquid droplets from the nozzles in a state that a nozzle-formation surface of the head in which the nozzles are formed faces a surface of the recording medium on which the image is to be formed.
The image forming apparatus as described above is configured to include a tank in which liquid is stored and a channel which communicates the tank and the head as a means for supplying the liquid to the head, so that the liquid in the tank is supplied to the nozzles of the head. In order to prevent the liquid from leaking from the nozzles when image formation is not performed, the image forming apparatus is configured such that a liquid surface of the tank is lower than liquid surfaces of the nozzles and negative pressure is applied to the liquid surfaces of the nozzles due to difference between the liquid surface of the tank and the liquid surfaces of the nozzles (water head difference).
In this type of image forming apparatus, a transport path of the recording medium extending in a horizontal direction is included therein, the head is mounted above the recording medium transported in the horizontal direction, and the image formation is performed by jetting the liquid downward from the head. Therefore, in this type of image forming apparatus, a casing extending in the transport direction of the recording medium along a horizontal plane is often adopted. Although the apparatus having such a casing is stably installed, the apparatus has a problem such that an installation area is large. In view of this, in order to reduce the installation area, there has been also suggested an image forming apparatus in which the transport direction of the recording medium extends in a vertical direction to perform the image formation with respect to the recording medium transported in the vertical direction. In particular, the recording medium is transported in the vertical direction, and the nozzle formation surface parallel to the vertical plane is provided in the head to face the recording medium. The plurality of nozzles are formed in the nozzle formation surface in a state of being aligned in the vertical direction. The image formation on the recording medium is performed as follows. That is, the liquid droplets are jetted from the nozzles with respect to the recording medium transported in the vertical direction while the head is reciprocated in the horizontal direction (width direction of the recording medium). In the apparatus having such a construction, it is possible to reduce the installation area by reducing the width of the casing or the dimension in a depth direction.
However, since the plurality of nozzles are aligned in the vertical direction, the nozzles have positions in a height direction different from one another. In this case, the liquid is less likely to be supplied as the position of the nozzle is higher due to the influence of the water head difference, and a size of each of the liquid droplets jetted tends to decrease gradually. In a case that the difference between the size of the liquid droplet jetted from the uppermost nozzle and the size of the liquid droplet jetted from the lowermost nozzle is large in one head, there is fear that unevenness in concentration is caused in the formed image and the image is deteriorated.
In order to address the problem as described above, Japanese Patent Application laid-open No. 2002-205389 has suggested a printing apparatus having a construction described below. That is, the printing apparatus includes a head of serial type (serial type head), which faces a transport route of a recording medium having difference in height and performs image recording while scanning in the horizontal direction. In the printing apparatus, transport of the recording medium is controlled such that an image area formed by liquid droplets jetted from the nozzles, of the nozzles aligned in the vertical direction of the head, disposed on an upper half portion and an image area formed by liquid droplets jetted from the nozzles, of the nozzles aligned in the vertical direction of the head, disposed on a lower half portion are substantially overlapped with each other, and the head is driven to form a desired image by overlapping the two image areas. In particular, at first, the liquid droplets are jetted from the nozzles, of the nozzles aligned in the vertical direction, disposed on the lower half portion, during one scanning of the head. In this situation, the head jets the liquid droplets on every other pixel with respect to the scanning direction. Thereafter, the recording medium is transported so that the nozzles disposed on the upper half portion face the recording medium at positions at which the liquid droplets jetted from the nozzles disposed on the lower half portion have been landed. Then, the liquid droplets are jetted from the nozzles disposed on the upper half portion onto the recording medium at positions interposed between pixels formed by the liquid droplets jetted from the nozzles disposed on the lower half portion.
However, since influences of the difference in height of nozzles disposed on the lower half portion and the difference in height of nozzles disposed on the upper half portion are not considered in the method described in Japanese Patent Application laid-open No. 2002-205389, unevenness in concentration is caused.
In view of the above problem, an object of the present teaching is to suppress generation of unevenness in concentration and deterioration in image quality in an image forming apparatus provided with a head, in which a plurality of nozzles are formed to have positions in a height direction different from one another.